Lately i've had : 8800GTX, 9800GX2, GTX 260, GTX 280, 4870, and now 4870X2
The 280 and X2 will run any game maxed out with AA + AF on my 2560x1600 monitor (bar Crysis). They will
easily cope with 1920x1200.
But for that res i'd go for a GTX 260 896MB, as 512MB will soon not be enough for that res - i'm always running out of memory on 512MB cards at 2560x1600, and can run into it @ 1920x1200 on a handful of games with crazy amounts of AA + AF. When you run out of memory things run seriously poor.
Plus the GTX 260 overclocks really well -
see here. It will atleast match a 4870 if you clock it.
If you dont wonna overclock it, then for a bit more the 1GB 4870 would be good, or the 280 would be great. Whatever you do i'd really recommend
not getting a card with only 512MB for a high res monitor, i'm talking from a lot of expericence here. I've also found games load quicker on cards with more than 512MB.